The development of new solid-state laser materials for mid-infrared (mid-IR) laser sources continues to be interest for potential applications in remote sensing of bio-chemical agents, IR countermeasures, and IR spectroscopy. Fluorescent materials based on Ho3+ doped crystals and glasses with narrow phonon spectra cover a wide wavelength range between ~1-4 µm. In this work, spectroscopic characterization on infrared emission properties of trivalent holmium (Ho3+) doped potassium lanthanum chloride (K2LaCl5) were explored. K2LaCl5 is slightly hygroscopic but possesses a maximum phonon energy of 235 cm-1. The low maximum phonon energy of K2LaCl5 leads to low non-radiative decay rates and efficient IR fluorescence. The studied Ho3+ doped K2LaCl5 material was grown by Bridgman technique. Using ~900 nm excitation, IR emissions centered at ~1.66, ~1.995, and ~3.90 µm were observed from Ho:K2LaCl5 corresponding to the 5I5-->5I7, 5I7-->5I8, and 5I5-->5I6 transitions of Ho3+ ions. Spectroscopic results and data modeling including the Stark level energies, Judd-Ofelt analysis, transitions cross-sections, and fluorescence dynamics will be presented at the conference.
Gamma rays produced passively by cosmic ray interactions and by the decay of radioelements convey information about the elemental makeup of planetary surfaces and atmospheres. Orbital missions mapped the composition of the Moon, Mars, Mercury, Vesta, and now Ceres. Active neutron interrogation will enable and/or enhance in situ measurements (rovers, landers, and sondes). Elemental measurements support planetary science objectives as well as resource utilization and planetary defense initiatives. Strontium iodide, an ultra-bright scintillator with low nonproportionality, offers significantly better energy resolution than most previously flown scintillators, enabling improved accuracy for identification and quantification of key elements. Lanthanum bromide achieves similar resolution; however, radiolanthanum emissions obscure planetary gamma rays from radioelements K, Th, and U. The response of silicon-based optical sensors optimally overlaps the emission spectrum of strontium iodide, enabling the development of compact, low-power sensors required for space applications, including burgeoning microsatellite programs. While crystals of the size needed for planetary measurements (>100 cm3) are on the way, pulse-shape corrections to account for variations in absorption/re-emission of light are needed to achieve maximum resolution. Additional challenges for implementation of large-volume detectors include optimization of light collection using silicon-based sensors and assessment of radiation damage effects and energetic-particle induced backgrounds. Using laboratory experiments, archived planetary data, and modeling, we evaluate the performance of strontium iodide for future missions to small bodies (asteroids and comets) and surfaces of the Moon and Venus. We report progress on instrument design and preliminary assessment of radiation damage effects in comparison to technology with flight heritage.
The scintillators currently providing the best energy resolution lower than 2.6% at 662 keV and sizes larger than 1 in. dia. 1 in. height are LaBr3(Ce) and SrI2(Eu). Despite energy resolution and decay time performance of LaBr3(Ce), the intrinsic radioactivity, due to naturally occurring 138La isotope in the matrix is a limitation for low count rate applications such as radioisotope identification of weak sources. Cesium Hafnium Chloride (CHC) is a high effective atomic number (Zeff=58) moderate density (3.86 g/cm3) scintillator for gamma spectroscopy, offering a cubic crystal structure, no intrinsic radioactivity, and highly proportional light yield, without intentional doping. CHC boasts a cubic crystal structure that is isostructural to K2HfCl6 and analogous to calcium fluoride with cesium ions in the fluorine ion position and the [HfCl6]2- octahedral replacing calcium ions. The scintillation of CHC is centered at 400 nm, with a principal decay time of 4.37 s, a light yield of up to 54,000 photons/MeV and energy resolution of 3.3% at 662 keV and we report on the effects of doping on the scintillation properties of CHC.
Alkaline-earth scintillators such as strontium iodide and other alkaline-earth halides activated with divalent europium represent some of the most efficient and highest energy resolution scintillators for use as gamma-ray detectors in a wide range of applications. These applications include the areas of nuclear nonproliferation, homeland security, the detection of undeclared nuclear material, nuclear physics and materials science, medical diagnostics, space physics, high energy physics, and radiation monitoring systems for first responders, police, and fire/rescue personnel. Recent advances in the growth of large single crystals of these scintillator materials hold the promise of higher crystal yields and significantly lower detector production costs. In the present work, we describe new processing protocols that, when combined with our molten salt filtration methods, have led to advances in achieving a significant reduction of cracking effects during the growth of single crystals of SrI2:Eu2+. In particular, we have found that extended pumping on the molten crystalgrowth charge under vacuum for time periods extending up to 48 hours is generally beneficial in compensating for variations in the alkaline-earth halide purity and stoichiometry of the materials as initially supplied by commercial sources. These melt-pumping and processing techniques are now being applied to the purification of CaI2:Eu2+ and some mixed-anion europium-doped alkaline-earth halides prior to single-crystal growth by means of the vertical Bridgman technique. The results of initial studies of the effects of aliovalent doping of SrI2:Eu2+ on the scintillation characteristics of this material are also described.
Impurity analysis and compositional distribution studies have been conducted on a crystal of
LiInSe2, a compound semiconductor which recently has been shown to respond to ionizing radiation.
IR microscopy and laser induced breakdown spectroscopy (LIBS) revealed the presence of
inclusions within the crystal lattice. These precipitates were revealed to be alkali and alkaline earth
elemental impurities with non-uniform spatial distribution in the crystal. LIBS compositional maps
correlate the presence of these impurities with visual color differences in the crystal as well as a
significant shift of the band gap. Further, LIBS revealed variation in the ratio of I-III-VI2 elemental
constituents throughout the crystal. Analysis of compositional variation and impurities will aid in
discerning optimal synthesis and crystal growth parameters to maximize the mobility-lifetime
product and charge collection efficiency in the LiInSe2 crystal. Preliminary charge trapping
calculations have also been conducted with the Monte Carlo N-particle eXtended (MCNPx) package
indicating preferential trapping of holes during irradiation with thermal neutrons.
Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching
ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the
electron track. For example, the fraction ηeh of excitations that are free carriers versus excitons was shown by Payne and
coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence
studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some
assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based
excitation density n (eh/cm3) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius
and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have
recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However,
experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed
difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and
transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for
modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional
parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response
from independently determined material parameters is becoming possible on an increasingly firmer data base. This
paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.
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